The present invention relates to a thermosetting powdery coating composition having excellent dispersibility of pigments and workability in coating works and capable of forming a coating film having excellent surface luster, insusceptibility to stains, impact resistance, weatherability and the like.
As is known, the techniques of powder coating are widely practiced in recent years for coating of metal-made bodies in general by virtue of the advantages in respect of material saving and energy saving as well as absence of problems due to environmental pollution. In particular, the techniques of powder coating are useful for coating of various substrate bodies of which high weatherability of the coating films is essential such as transportation and living structures, e.g., bridges, parapets, gates, fences, sidings of houses and the like, bodies and parts of automobiles, electric applicances and so on.
Powdery coating compositions used for powder coating are classified into thermoplastic and thermosetting powder coating composition depending on the type of the polymeric resin as the vehicles. For example, Japanese Patent Kokai 61181567, 61-181571 and 61-181572 disclose a thermoplastic powder coating composition of which the vehicle resin is a thermoplastic fluorocarbon resin such as a copolymer of ethylene and tetrafluoroethylene.
Several kinds of thermosetting powdery coating compositions used for powder coating are also known including polyester resinbased powdery coating compositions using a blocked isocyanate compound, triglycidyl isocyanurate and the like as the curing agent and powder coating compositions based on acrylic resins having glycidyl groups and using a dibasic acid as the curing agent (see, for example, "Funtai to Kogyo", February, 1984 issue, pages 33 to 42).
Though excellent in the weatherability of the coating films, the first mentioned thermoplastic fluorocarbon resin-based powdery coating compositions have several disadvantages that, in addition to the poor dispersibility of pigments, surface luster of the coating films and adhesion of the coating film to the substrate surface, the coating composition must be heated in the coating works to a temperature higher than the high melting point of the fluorocarbon resin consequently with poor workability and consumption of a large quantity of thermal energy.
The thermosetting powder coating compositions such as the above mentioned polyester resin-based and acrylic resin-based ones, on the other hand, are defective due to the relatively low weatherability of the coating films although they are free from the above mentioned problems in the thermoplastic fluorocarbon resin-based powdery coating compositions.